That's the big problem with the GPL: Bottom-feeders can take GPL code and
resell it, giving nothing to the developers.

So long as the seller complies with the GPL and does not remove copyright
notices, there's not a damned thing that can be done (at least as far as I
understand it).

If the Rockbox team didn't want the code resold by such low-life scum, then
they should have created a license which prohibited the for-profit resale of
the Rockbox firmware, documentation, and source code.